This invention relates to the domain of the design and manufacture of filter assemblies used to filter fluids circulating in motors or hydraulic equipment.
Such filter assemblies are composed of a filter body inside which a filter cartridge is arranged generally with a cylindrical shape and that includes a filter medium, for example made of paper, cardboard or felt. This filter medium is conventionally delimited by two end plates.
Two cases can arise.
Either the filter cartridges cooperate with a central tube that may or may not be fixed to the end plates (the case in which the cartridge is fixed to a tube designed to cooperate with another tube permanently installed in the filter assembly also being possible), the outside diameter of the central tube being similar to the inside diameter of the filter medium so as to limit deformations of the medium under the effect of pressure applied inside the filter assembly.
Or the cartridges are inserted in a tube in which the inside diameter is similar to the outside diameter of the filter medium, to limit deformations of the medium due to passage of the fluid from the central space of the medium towards the outside of the medium.
According to one widespread technique, filter assemblies use a support tube on which the filter cartridge is added, the tube being permanently installed in the filter assembly.
According to yet another technique, cartridge bottom end plates are extended to provide cartridge placement and support means inside the filter assembly, in this case the central tube being installed and removed together with the cartridge.
The invention is indifferently applicable to the different configurations that have just been mentioned, but is more particularly applicable to filter assemblies in which the tubes are installed permanently.
Conventionally, several considerations need to be taken into account in the design of a support tube:                the filter medium support function, that must be provided in an optimised manner (in general, the tube must be as close as possible to the medium);        a low “head loss” or fluid pressure loss during the passage through the tube and through the medium;        minimum friction and/or interference with the seals of the filter cartridge during installation or replacement of the filter cartridge on the tube.        
Solutions according to prior art particularly include filter assemblies as described in document DE—44 30 341 using a support tube with an irregular helical ribbed pattern.
Such a tube has the following disadvantage.
When the filter cartridge is put into place on the support tube already present in the filter assembly, the rib of the tube generates resistance to the tube being inserted as it passes inside the pre-installed seal on the bottom and top end plates of the cartridge. The same is true when the cartridge is extracted for replacement.
The seal, designed to be compressed on the tube in the operating position, tends to deform as a function of the few support points that it will have on the tube and to have chords such that the inscribed diameter in these chords is less than the diameter of the tube.
The phenomenon may be marginal when a felt seal is used, but it is more pronounced and more damaging if the seal is flexible (elastomer or other) which is tending to become more frequent.
Since filter cartridges used at the moment are expected to keep the fluid increasingly clean, it is no longer acceptable to risk damaging the seal when the cartridge is installed on the tube.
Another solution has been proposed in prior art in which the tube has two series of variable sloped helical ribs, each series extending for 180° around the tube, and the two series together forming a spiral.
However, the spiral in question is not absolutely continuous.
The ribs in each series have a variable slope which reduces to substantially zero where they join together with the ribs in the other series.
Due largely to manufacturing constraints (and particularly moulding constraints), there is often a slight offset at the point at which the ribs in one series join the ribs in the other series, which introduces a discontinuity in the spiral formed by the two series of ribs.
In some cases, this can result in the occurrence of moulding burrs near the offset area between the ribs.
These defects (offset, burrs) tend to generate unwanted friction phenomena on the seals of the filter cartridge during installation or replacement of the filter cartridge.
Furthermore, the arc along which the seal is no longer supported by the tube is always too long.